This invention relates to analog-to-digital signal converters, and particularly relates to sigma-delta analog-to-digital converters.
Sigma-delta analog-to-digital converters permit high speed digital signal processing of analog signals. Such converters achieve high resolution through oversampling and noise-shaping. With the development of faster and more complex digital very large scale integrated circuit (VLSI) technology, digital signal processing has become increasingly important in a growing number of applications. Highly developed digital signal processing functions are now being incorporated into inexpensive consumer electronics to process signals at audio and video frequencies. Digital signal processing has advantages in precision, flexibility, and programmability when compared with traditional analog signal processing techniques. Digital signal processing may be able to provide these same advantages to signal processing at microwave frequencies, but the difficulty of digitizing such signals with available analog-to-digital converters typically precludes such processing. For example, a flash type semiconductor analog-to-digital converter is capable of digitizing signal frequencies above 1 GHz, but typically achieves less than 7 bits of resolution for such frequencies. This rather limited resolution is generally unacceptable for most radio systems which require a high dynamic range. There is a need for analog-to-digital converters that are able to digitize microwave signals with a high dynamic range.
Sigma-delta analog-to-digital converters are a class of analog-to-digital converters that achieve high resolution through over-sampling and noise shaping. Sigma-delta converters typically include an integrator, a single rough quantizer, a digital filter, and a feedback loop. The input analog signal is applied to the integrator. The quantizer operates at high speed to convert the output of the integrator to a single bit digital signal. The digital filter converts the high speed single bit output of the quantizer into a multi-bit digital output. The feedback loop includes the quantizer, a digital-to-analog converter and the integrator. The feedback in a delta-sigma converter integrates the error in the least significant digit thereby shifting any noise to higher frequencies above the fundamental frequency of the analog input signal. The effect of this is to shift most of the qauntization noise to frequencies above the bandwidth of the analog input signal.
Through this quantization noise shaping, a high signal to noise ratio is achieved in the baseband. The digital filter is a lowpass filter having a sharp cutoff above the bandwidth of the analog input signal. This digital filter eliminates the high frequency quantization noise and produces a high resolution multi-bit output at a reduced sampling rate, which is typically the Nyquist rate. An important advantage of delta-sigma analog-to-digital converters is that this resolution is achieved without the need to precisely match analog components with one another.
Semiconductor sigma-delta analog-to-digital converters were originally developed for digitizing baseband signals, such as audio signals. Since semiconductor sigma-delta analog-to-digital converters typically operate at clock frequencies below 100 MHz, and a large over-sampling ratio (i.e., the sampling rate to the Nyquist rate) is required to obtain high resolution, semiconductor sigma-delta analog-to-digital converters have traditionally been employed to digitize analog signals below 1 MHz. Bandpass sigma-delta modulators for analog-to-digital conversion have been proposed. A bandpass sigma-delta modulator is designed to suppress quantization noise over a narrow band of frequencies centered at an intermediate frequency, which may be a significant fraction of the clock frequency. Bandpass sigma-delta modulators may, therefore, digitize narrowband signals, such as radio frequency signals, with a high signal to noise ratio and a large dynamic range. Because the clock frequency of semiconductor bandpass sigma-delta converters is limited to about 100 MHz, the center frequency of the analog input signal is typically in the tens of MHz.
If microwave signals are to be processed by sigma-delta modulators for analog-to-digital converters, then multi-GHz clock speeds are needed. Superconducting technology, having clock rates over 100 GHz., provides promise for this application. Also, Josephson junctions, which naturally generate qauntized voltage pulses, have been found to provide excellent performance in sigma-delta modulators. Because the area of each pulse is equal to the magnetic flux quantum (.PHI..sub.0 =h/2e), these pulses are known as single flux quantum (SFQ) pulses. The performance depends on the ability of the quantizer to produce an output pulse with an accurate and stable area.
Superconductive delta-sigma analog-to-digital converters typically employ a superconducting quantum interference device (SQUID) to generate pulses at frequencies of up to 30 GHz. U.S. Pat. No. 5,341,136 discloses a bandpass sigma-delta modulator for an analog to digital converter that includes, with reference to FIGS. 3 and 4 thereof, a feedback path 38 from the output to the pulse generator 40. This feedback path is disclosed to generate M feedback pulses for each SFQ pulse generated by the Josephson junction 34. Because Josephson devices have rather low gain, complex circuits using many Josephson junctions may be needed to implement a pulse generator with a significant value of M. U.S. Pat. Nos. 5,198,815 and 5,327,130 also disclose sigma-delta modulators for analog-to-digital converters that employ feedback pulse generators.
An active feedback loop will diminish circuit speed, particularly at high frequencies. Due to inevitable gain-bandwidth tradeoffs, the circuit delays added by feedback pulse generators such as disclosed in U.S. Pat. No. 5,341,136 may be substantially greater than the switching time of the Josephson junction quantizer. Consequently, the maximum clock frequency of the sigma-delta modulator will be limited significantly by the feedback loop. Such limitations in clock speed are particularly detrimental to the performance of a sigma-delta modulator for an analog-to-digital converter since the resolution is a function of the sampling rate to the Nyquist rate.
It is desirable to have a bandpass sigma-delta modulator whose clock speed is minimally limited, and permits digitization of high frequency signals with high resolution.